New functional parts to solve a particular problem are now frequently designed and modeled by rapid prototyping. This entails generating physical models directly from a 3-D computer drawing created with computer aided design software. The model design is then electronically transmitted to a rapid prototyping system. Stereolithography (SL) is such a system.
The stereolithography apparatus (SLA) consists of a vat of a liquid polymer in which there is a movable elevator table/platform that is capable of moving (i.e. lowering) in very precise increments, the increments depending on the requirements defining the type of model to be constructed. A helium/cadmium laser is then used to generate a small but intense beam of ultraviolet light that is moved across the top of the vat of liquid polymer by a computer-controlled optical scanning system. At the point where the laser beam meets the polymer, the polymer is changed into a solid. As the laser beam is directed across all surfaces of the three dimensions, the model is formed as a plastic object point by point and layer by layer. As each layer is formed, the elevator platform is lowered by the pre-determined increment, so that the next layer can be scanned in. As each additional layer is formed, it bonds to the previous one. What results is a model generated by a precise number of successive layers.
After the model is removed from the SLA, it is ultrasonically cleaned to remove any excess polymer from crevices and openings. Then the model undergoes a curing operation to finish hardening the polymer. The curing operation usually involves bathing the model in intense long-wave ultraviolet light which causes any uncured liquid polymer that may be trapped within the structure to harden. When the model is properly cured, the surface can be finished in a number of ways to meet the requirement.
Under the current practice, stereolithography process is used to produce a mold or other inoperative prototype which is then used as a master from which to fabricate functional parts using conventional casting or machining technology. The materials normally used to produce such master parts via SL process have special properties that render them suitable for SL but not for fully functional, flightweight parts, such as missile hardware.
An important piece of missile hardware is the igniter to boost the motor of the missile. Igniter 100 is normally positioned in throat 403 of nozzle 106 of missile 101, as illustrated in FIG. 1, and boosts motor 102 by directing burning pyrotechnics onto propellant fuel 104. Due to the limitations of conventional manufacturing processes and the complexity of removable missile igniters, typically such an igniter was made in several separate pieces that were then put together to form an assembly. In a conventional removable igniter, frangible fingers or tabs are used to hold the igniter secure in the missile nozzle until sufficient pressure builds inside the missile to eject the igniter. For the frangible tabs to work, a screw-in sleeve must be inserted into the throat of the igniter, thereby forcing the tabs into securing positions in nozzle throat 403. This technique is not desirable from a safety standpoint as it requires as least two parts of a fully loaded igniter to be rotated, screwed or otherwise moved, presenting a hazard of untimely ignition due to friction between moving parts, handling or electrostatic discharge.